1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method for obtaining an ultrasonic image by transmission and reception of an ultrasonic wave, and more particularly, to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method for displaying a blood flow image and a Doppler spectrum image simultaneously or almost simultaneously at least.
2. Description of the Related Art
In an observation of an object using an ultrasonic diagnostic apparatus, an operation mode called a triplex mode is used. The triplex mode is a composite mode for displaying a two dimensional configuration image, a two dimensional blood flow image, and a Doppler spectrum image simultaneously or substantially simultaneously.
The two dimensional configuration image is a configuration image generated by converting the intensity of reflected ultrasonic waves to luminance. The two dimensional blood flow image is a two dimensional color image of blood flow generated by extracting a Doppler shift (frequency shift) caused by blood flow, and is overlaid on the two dimensional configuration image by color mapping (color flow mapping). The Doppler spectrum image is a graph showing a change over time in Doppler shift of ultrasonic waves caused by blood flow.
That is, in the triplex mode, a position of a blood vessel and blood flow therein are specified in a two dimensional configuration image and a two dimensional blood flow image, and a blood flow rate is measured in a Doppler spectrum image. In order to display the two dimensional blood flow image and the Doppler spectrum image substantially simultaneously, transmission/reception of ultrasonic waves to obtain the two dimensional blood flow image and transmission/reception of ultrasonic waves to obtain the Doppler spectrum image need to be alternately performed. As a scanning method in the triplex mode, Doppler interleaved scanning is known (see, for example, Japanese Patent Application (Laid-Open) No. 6-7348).
The Doppler interleaved scanning is a method. for scanning an object by alternately performing scanning for a two dimensional configuration image (hereinafter called “B mode scanning”) or scanning for a color two dimensional blood flow image (hereinafter called “CFM mode scanning”) and scanning for a Doppler spectrum image (hereinafter called “D mode scanning”) on a one-by-one basis over time. In the CFM mode scanning, transmission/reception of ultrasonic waves is performed a plurality of times sequentially in the same scanning line in order to remove a clutter component. The number of repetitions of transmission/reception of ultrasonic waves in the same scanning line is called a number of ensemble.
In the Doppler interleaved scanning, B mode scanning or CFM mode scanning is sandwiched between D mode scannings. Therefore, in transmission/reception of ultrasonic waves performed in D mode scanning, pulse-repetition frequency (DpPRF) cannot be set at high. When the DpPRF cannot be set at high, aliasing occurs in the Doppler spectrum image when high-rate blood flow is detected.
In order to overcome this problem, a scanning method called “Doppler segmented scanning” may be used. In this scanning method, B mode scanning or CFM mode scanning and D mode scanning are alternately performed over time in respective groups of beams. In other words, in the Doppler segmented scanning, transmission/reception of ultrasonic waves is alternately performed while distinguishing a B/CFM segment where B mode scanning or CFM mode scanning is continuously performed for a predetermined time period and a D segment where D mode scanning is continuously performed for a predetermined time period. By repeating these two segments, one frame of a two dimensional configuration image is created (see, for example, Japanese Patent Application (Laid-Open) No. 2002-143168).
In the Doppler segmented scanning, DpPRF can be set at high in a segment of D mode scanning. Therefore, aliasing does not occur even in a high-rate blood flow and a favorable waveform can be obtained in a Doppler spectrum image.
When the Doppler segmented scanning is used in the triplex mode, a time ratio between a B/CFM segment and a D segment must be 1:2 or more. In the B/CFM segment, D mode scanning is not performed and thus a gap occurs in a Doppler spectrum image during this time period. This gap is complemented by using data collected in the D segment. Considering a complementation accuracy of the Doppler spectrum image, the time ratio should be 1:2 or more.
However, when the time ratio is 1:2 or more, the frame rate is ⅓ or less compared to that in a composite mode of a two dimensional configuration image and a two dimensional blood flow image. This is because, since neither B mode scanning nor CFM mode scanning is performed in a D segment, the frame rate decreases in accordance with the applied D segment.
Consequently, in the triplex mode, high-rate blood flow can be detected by using the Doppler segmented scanning but an increase in complementation accuracy of a Doppler spectrum image and an increase in frame rate cannot be achieved at the same time.
That is, in an ultrasonic diagnosis apparatus that displays at least a two dimensional blood flow image and a Doppler spectrum image simultaneously or substantially simultaneously, it is difficult to achieve the following three elements at the same time: an adequate detection of high-rate blood flow; an increase in complementation accuracy of a Doppler spectrum image; and an increase in frame rate.